1. Field of the Invention
The present invention relates to noiseless covers such as oil pans, cylinder head covers, front covers and the like, adapted to restrain noise from engines.
2. Description of the Prior Art
In usual engines for automobiles, covers such as oil pans, cylinder head covers, front covers and the like are secured to cylinder blocks and cylinder heads constituting engine bodies. These covers are, however, aluminum casts or steel plates formed by pressing which have generally poor damping capacity to increase noise radiation coefficient (a ratio of noise energy to received vibration energy) which provide a problem for engine noise. Particularly, oil pans are mostly of mild steel plates formed by pressing, so that they tend to cause surface vibrations by applied vibrating forces. In addition, resonance frequencies of the surface vibrations are in medium frequency zones which are the most acute zones in the problem of engine noise, with the result that the radiation of the engine noise greatly increases. For example, the noise from an oil pan is often as much as more than 15% of all the noise from an engine, and it becomes more than 20% in case of a deep oil pan.
In order to prevent such a noise from covers, one solution is to make the covers thicker to restrain surface vibration thereof. On the other hand, however, the thicker covers increases vibrations of higher frequencies and total weights of engines.
To solve this problem, a noiseless cover as shown in FIG. 1 has been proposed, wherein a mounting member 6 embracing an elastic material 5 connects part of a cylinder block 1, for example, a bearing cap as shown with a mid-portion of an oil pan 3 whose peripheral portions are fixed to the cylinder block 1 of an engine body. The mounting member 6 is generally a usual antivibration type. With this arrangement, the mid-portion of the oil pan is resiliently supported through the mounting member 6 by the cylinder block 1, so that even if the vibration is transmitted from the engine body to the oil pan, the vibration would be absorbed or damped by the mounting member 6, thereby preventing the surface vibration of the oil pan, whereby the radiation of noise is therefore prevented.
With this arrangement, however, the mounting member 6 must be incorporated in place simultaneously in mounting the oil pan on the engine body, thereby making difficulty the mounting of the oil pan. In addition, with this arrangement the mounting member 6 gives rise to a further problem to ensure a sealing of the oil pan at the mounting member 6.